The instant invention involves a method of producing multilayer capacitors provided with end terminations. In particular this invention relates to a method of producing base metal electrode multilayer capacitors having metallized end terminations of increased strength.
As shown in H. J. Hagemann et al, Philips Tech. Rev. 41, 89-98, 1983/84, No. 3, a multilayer capacitor consists of a ceramic body in which there are embedded electrode layers and which electrode layers are alternately connected to metallized end terminations on opposite faces of the capacitor. Through this arrangement, the individual capacitors formed by ceramic layers between the electrodes are all connected in parallel. As a result, the capacitance adds up so that these capacitors can be made with a high capacitance per unit volume.
Due to the presence of the metallized end terminations at the faces of the ceramic bodies, these capacitors function as "chip components" which can be soldered directly to the conducting paths of a printed circuit board and are particularly adapted for automatic mounting in printed circuit boards.
As described in the Hagemann et al article the manufacture of the multilayer capacitors is as follows: A ceramic dielectric material, particularly a barium titanate or a modified barium titanate, is mixed with a binder to form a suspension. Strips or tapes are cast from the suspension. The strips or tapes are then cut into sheets and a metal paste is applied onto the sheets, in a repetitive pattern, by screen printing. The sheets with the resultant printed electrodes on them are stacked so that the screen printed metal layers are interposed between layers of the ceramic material and the stacked sheets are consolidated under pressure. Individual consolidated units are then cut apart, heated to burn off the binder, and fired to produce a coherent integral structure of ceramic and metal.
During the screening, stacking and cutting procedures, the sheets are aligned so that the edges of the electrodes alternatively extend to two opposite faces of the unit. These two faces are then metallized as a result of which the individual ceramic capacitors in each integral stack are all connected in parallel.
As is further pointed out in this article, the dielectric ceramics generally employed normally require firing in air at temperatures between 1200.degree. C. and 1400.degree. C. As a result, only nonoxidizing metals having a high melting point are useful as the electrodes of these multilayer capacitors. In practice these metals are limited to the very expensive metals palladium gold or platinum and their silver alloys. As a result of this, these capacitors are very costly and cannot be readily used as components in an automatic chip mounting system and in particularly in mass produced consumer articles. To overcome this problem this article suggests the use of a base metal, particularly nickel, for the electrodes.
However, use of a nickel electrode is not suitable under normal firing conditions as such firing conditions cause oxidation of the nickel resulting in undesired reactions between the nickel oxide and the ceramic materials as well as loss of electrical integrity. As a result of such reactions there is a strong tendency for delamination of the multilayer structure to occur. To prevent this oxidation, this firing is carried out in an atmosphere that is sufficiently reducing to prevent oxidation of the nickel.
However, as a result of the use of the reducing atmosphere, the barium titanate which is generally used as a ceramic material becomes semiconducting due to the loss of oxygen. To overcome this, acceptor-type dopants are added to the barium titanate. In particular ions of the transition metals Cr, Fe, Ga, Mn, and Ni are employed.
Conventionally, end terminations are formed by dipping the fired part in a paste consisting of a well dispersed mixture of silver, glass frit, and an organic binder and solvent mixture. The dipped parts are then dried and fired in air at a temperature in excess of 750.degree. C. The high temperature firing is required to provide a proper densification of the silver paste, adherence of the glass of the paste to the ceramic and to provide a good electric contact between the nickel electrodes and the terminations.
However, the firing at a high temperature in an oxidizing atmosphere, causes a certain amount of oxidation of the nickel electrodes. As a result capacitance is lost and the dissipation factor increases. Alternative terminations which are fired under reducing conditions prevent nickel oxidation, however, because the glasses used cannot contain reducable oxides such as Bi.sub.2 O.sub.3, PbO, or CdO, the strength of such terminations is low.
In an attempt to overcome this problem, Hurley U.S. Pat. No. 3,809,973 proposes the use of a thin film of gold on the faces having exposed nickel electrode edges before applying a silver paste to these faces and then heating the body in air to about 800.degree. C. Due to the presence of the gold layer, oxidation of the nickel electrode is inhibited. However, while the method of this patent solves the problem of the oxidation of the nickel electrodes, use of the expensive gold and elaborate processing requirements inhibit commercial use of this method particularly for production of multilayer capacitors for consumer products.
It is therefore an object of the instant invention to provide a less expensive method of increasing the strengths of the end metallizations of base metal electrode multilayer capacitors.
This and other objects of the invention will be apparent from the description that follows.